The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a polishing pad having a grooved pattern for a chemical mechanical polishing apparatus.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. Therefore, there is a need to periodically planarize the substrate surface to provide a flat surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing surface.
A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be supplied to the polishing pad to provide an abrasive chemical solution at the interface between the pad and the substrate. CMP is a fairly complex process, and it differs from simple wet sanding. In a CMP process, the reactive agent in the slurry reacts with the outer surface of the substrate to form reactive sites. The interaction of the polishing pad and abrasive particles with the reactive sites on the substrate results in polishing of the substrate.
An effective CMP process not only provides a high polishing rate, but also provides a substrate surface which is finished (lacks small-scale roughness) and flat (lacks large-scale topography). The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad. The polishing rate sets the time needed to polish a layer. Because inadequate flatness and finish can create defective substrates, the selection of a polishing pad and slurry combination is usually dictated by the required finish and flatness. Given these constraints, the polishing time needed to achieve the required finish and flatness sets the maximum throughput of the CMP apparatus.
A recurring problem in CMP is non-uniformity of the polishing rate across the surface of the substrate. One source of this non-uniformity is the so-called "edge-effect", i.e., the tendency for the substrate edge to be polished at a different rate than the center of the substrate. Another source of non-uniformity is termed the "center slow effect", which is the tendency of center of the substrate to be underpolished. These non-uniform polishing effects reduce the overall flatness of the substrate and the substrate area suitable for integrated circuit fabrication, thus decreasing the process yield.
Another problem relates to slurry distribution. As indicated above, the CMP process is fairly complex, requiring the interaction of the polishing pad, abrasive particles and reactive agent with the substrate to obtain the desired polishing results. Accordingly, ineffective slurry distribution across the polishing pad surface provides less than optimal polishing results. Polishing pads used in the past have included perforations about the pad. These perforations, when filled, distribute slurry in their respective local regions as the polishing pad is compressed. This method of slurry distribution has limited effectiveness, since each perforation in effect acts independently. Thus, some of the perforations may have too little slurry, while others may have too much slurry. Furthermore, there is no way to directly channel the excess slurry to where it is most needed.
Another problem is "glazing" of the polishing pad. Glazing occurs when the polishing pad is heated and compressed in regions where the substrate is pressed against the pad. The peaks of the polishing pad are pressed down and the pits are filled up, so the polishing pad surface becomes smoother and less abrasive. As a result, the polishing time increases. Therefore, the polishing pad surface must be periodically returned to an abrasive condition, or "conditioned", to maintain a high throughput.
In addition, during the conditioning process, waste materials produced by conditioning the pad may fill or clog the perforations in the pad. Perforations clogged with such waste materials do not hold slurry effectively, thereby reducing the effectiveness of the polishing process.
An additional problem associated with filled or clogged pad perforations relates to the separation of the polishing pad from the substrate after polishing has been completed. The polishing process produces a high degree of surface tension between the pad and the substrate. The perforations decrease the surface tension by reducing the contact area between the pad and the substrate. However, as the perforations become filled or clogged with waste material, the surface tension increases, making it more difficult to separate the pad and the substrate. As such, the substrate is more likely to be damaged during the separation process.
Yet another problem in CMP is referred to as the "planarizing effect". Ideally, a polishing pad only polishes peaks in the topography of the substrate. After a certain period of polishing, the areas of these peaks will eventually be level with the valleys, resulting in a substantially planar surface. However, if a substrate is subjected to the "planarizing effect", the peaks and valleys will be polished simultaneously. The "planarizing effect" results from the compressible nature of the polishing pad in response to point loading. In particular, if the polishing pad is too flexible, it will deform and contact a large surface area of the substrate, including both the peaks and the valleys in the substrate surface.
Accordingly, it would be useful to provide a CMP apparatus which ameliorates some, if not all, of these problems.